Device for determining the size distribution of aerosol particles

ABSTRACT

The invention relates to a device ( 30 ) for determining the size distribution of aerosol particles from a flow ( 11 ), the device ( 30 ) comprising an electric mobility analyzer ( 10 ) and an impactor ( 20 ), connected to each other in such a way that the nozzle part ( 22   a ) of the first stage of the impactor ( 20 ) is simultaneously the bottom plate of said mobility analyzer ( 10 ).

FIELD OF THE INVENTION

The invention relates to a device for determining the size distributionof aerosol particles .

BACKGROUND OF THE INVENTION

With tightening environmental regulations, there is an increasing needfor real-time measurement of particle emissions. In particular, the needfor measurement is present in the development of filtering methods, inthe research of various combustion processes, as well as in monitoringprocesses for actual emissions. For classifying the mobility sizedistribution of aerosol particles, real-time aerosol measurements applyvarious analyzers which measure the electric mobility of particles, suchas differential mobility analyzers (DMA).

FIG. 1 shows, in a simplified diagram, a mobility analyzer 10 accordingto prior art. The mobility analyzer 10 consists of an electricallyconductive, preferably cylindrical frame part 12, which is coupled to afirst constant potential, typically the ground plane, and thereby actsas an outer electrode. The ends of the frame part 12 are provided withcover and bottom plates 17 a and 17 b. An inner electrode 13 is placedcentrally in the frame part 12 and coupled via a power supply 15 to asecond constant potential. The power supply 15 is used to produce anelectric field between the frame part 12 of the device and the innerelectrode 13.

The cover and bottom plates 17 a and 17 b of the analyzer 10 areequipped with the necessary ducts to implement inlet and outlet pipes18, 14 and 16 as well as the couplings required by the inner electrode13.

So that particles could be separated on the basis of their electricmobility by means of the electric field, the aerosol particles to beintroduced into the analyzer must have an electric charge before theanalyzer 10. For this reason, the aerosol particles are typicallycharged by a separate charger (not shown in FIG. 1) before the analyzer10. The flow 11 coming from the charger is led via the inlet pipe 18 tothe analyzer. The flow 11 passes through the analyzer 10, primarilyexiting via the outlet pipe 16.

When entering the electric field between the frame part of the analyzer10 and the inner electrode 13, the charged aerosol particles in the flow11 are drawn, depending on the sign of the charge, either towards theframe part 12 or towards the inner electrode 13. Typically, mobilityanalyzers are implemented in such a way that interesting aerosolparticles are drawn towards the inner electrode 13.

The rate at which the aerosol particles drift towards the electrodesdepends on the electric mobility of the particles, which is dependent,in a known manner, on e.g. the mass and charge of the particle.

Particles having greater electric mobility move faster towards theelectrode determined by their charge, typically the inner electrode 13.As a result of this, the particles with greater electric mobility hitthe inner electrode 13 sooner than particles with smaller electricmobility. As the flow 11 passes towards the bottom plate 17 b of theanalyzer 10, particles with greater electric mobility hit closer to theend of the inner electrode 13 on the side of the cover plate 17 a, andparticles with smaller mobility hit closer to the end of the innerelectrode 13 on the side of the bottom plate 17 b. Those particles whoseelectric mobility is so small that they do not reach the inner electrode13 when moving with the flow 11 through the analyzer, are discharged viathe outlet pipe 16 from the analyzer.

Depending on the solution to be used, the above-presented mobilityanalyzer 10 can be implemented by a number of different ways. In itssimplest form, the solution does not apply the outlet pipe 14 placedinside the electrode 13 at all, wherein the complete flow 11 coming intothe analyzer 10 is discharged through orifices in the bottom plate 17 binto the outlet pipe 16. Thus, it is possible to use the analyzer toremove from the flow 11 the particles which have greater electricmobility than a certain value and which are deposited on the innerelectrode 13.

DMA analyzers are typically implemented in such a way that the innerelectrode 13 is provided with a narrow slit 19 which is coupled to thesecond outlet pipe 14. Thus, particles falling into the slit 19 areabsorbed into the second outlet pipe 14. If the analyzer 10 isconstructed in such a way that the flow 11 to be measured is introducedinto the analyzer 10 at a certain distance from the inner electrode 13,the voltage difference between the frame part 12 of the analyzer 10 andthe inner electrode 13 can be adjusted to determine the mobility rangeof the particles falling into the slit 19. Thus the DMA analyzer can beused to separate particles falling into a certain mobility range fromthe flow 11 under analysis, by guiding them into the second outlet pipe14, wherein particles with greater electric mobility adhere to the innerelectrode 13 before the slit 19 and particles with smaller mobility aredischarged with the flow along the outlet pipe 16.

Furthermore, it is known to implement the mobility analyzer 10 as amulti-channel differential mobility analyzer, in which the innerelectrode 13 of the mobility analyzer 10 is equipped with severaldetection surfaces which indicate the number of particles hitting them,for example on the basis of the charges transferred by the particleshitting them. The detection surfaces are preferably placed onto thesurface of the inner electrode 13 in such a way that each of themcollects particles falling into a certain mobility range. By monitoringthe signals given by these detection surfaces, it is possible tomeasure, in real time, the electric mobility of particles in the flow 11under analysis and, on the basis of this, to compute the sizedistribution based on the electric mobility diameter of the particles.

The mobility analyzers are most precise for particles with a small mass,because the electric mobility of the particles is inversely proportionalto the mass of the particle; that is, the greater the mass of theparticle, the smaller the mobility of the particle. In typical particlemeasurement environments, the aim is to measure particles whosediameters vary from some tens of nanometers to tens of micrometers. Inthis range, there are typically differences of several orders in themobility of the particles, wherein it is extremely difficult to measurethe whole range simultaneously by one device with a sufficiently highprecision. Electric mobility analyzers are primarily used in fineparticle analyses, in which the mobility analyzers are most precise, dueto the high mobility of the fine particles.

For measuring primarily larger particles, classification methods basedon the aerodynamic diameter of the particles are normally used, such asimpactors. The electrical low pressure impactor (ELPI) developed byDekati Ltd provides a solution for real-time measurement of aerosolparticles.

FIG. 2 shows a cross-sectional view of two upper stages 21 a and 21 b ofan electrical low pressure impactor 20, and chambers 29 a and 29 brelated to them. An air flow 11 to be analyzed is sucked by means of anunderpressure produced by a pump (not shown in FIG. 2) into theelectrical low pressure impactor 20 and into the first chamber 29 a ofthe impactor. Each stage comprises a nozzle part 22 a, 22 b, equippedwith orifices, through which the air flow 11 carrying particles flows.Collection surfaces 23 a, 23 b are placed behind the nozzle parts 22 a,22 b. Each collection surface 23 a, 23 b is equipped with at least oneoutlet 25, through which the flow 11 is allowed to flow to the nextchamber or out of the impactor. Insulators 24 a, 24 b, 24 c placedbetween the stages 20 a, 20 b insulate the different stages 20 a, 20 bfrom each other and the first stage of the impactor from the cover part26.

The direction of the air flow 11 flowing from the orifices of the nozzlepart 22 a, 22 b is abruptly changed when it meets the collection surface23 a, 23 b. The particles carried by the flow 11 and having asufficiently large aerodynamic particle size cannot follow the abruptchange in the direction of the flow, but they hit the collection surface23 a, 23 b, being deposited on the same. When hitting the collectionsurface 23 a, 23 b, the charged particles cause a change in the chargelevel of the collection surface 23 a, 23 b. As the collection surface 23a, 23 b is electrically coupled to said impactor stage 20 a, 20 b whichis, via an electrical coupling 27 a, 27 b further connected to a controlu charge level of the collection surface 23 a, 23 b is indicated as anelectric current which can be detected by sensitive current metersplaced in the control unit 28.

The above-described method makes it possible to classify the particlesselectively according to the size. By selecting, in a known way, thenumber and size of orifices in the nozzle part 22 a, 22 b, the distancebetween the nozzle part 22 a, 22 b and the collection surface 23 a, 23b, as well as the flow rate to be used, each impactor stage 20 a, 20 bcan be dimensioned so that only particles larger than a given particlesize are deposited on the collection surface at each stage. Bydimensioning successive impactor stages in such a way that particleswith different particle sizes are collected on different stages, thecurrents from the different impactor stages 20 a, 20 b measured by thecontrol unit 28 can be used to determine, in real time, the sizedistribution of the particles in the flow 11 to be measured, on thebasis of the aerodynamic diameter.

As a result of the operating principle of the impactor, the impactorsare most sensitive to particles whose mechanical mobility is small, i.e.typically particles with a large aerodynamic diameter. On the otherhand, it is difficult to measure small particles with the impactors,because, due to the high mechanical mobility of the particles, high flowrates and abrupt changes in the direction of flow are thus required toseparate the particles from the flow.

A problem with the above-presented real-time particle measurements ofprior art has been their inapplicability for measurements in a widerange of particle sizes. DMA analyzers are suitable for the measurementof particles with a small diameter and high electric mobility, andimpactors are suitable for the measurement of particles with a verylarge aerodynamic diameter.

In certain applications, it is advantageous to carry out the measurementof the particle size distribution within a wide range of particle sizes.If the measurement is made with an electric mobility analyzer, theproblem is the poor sensitivity of the device for large particles, andif an impactor is used, the problem is the detection of small particleswith a sufficient precision. Attempts have been made to solve thisproblem with prior art devices by carrying out the measurement in twoparts, wherein an electric mobility analyzer 10 is first used to measurethe size distribution of small particles. After this, the flow obtainedfrom the mobility analyzer 10 through the outlet pipe 16 is guided to aseparate impactor 20 to determine the size distribution of largerparticles.

A problem in the above-described solution of prior art lies in the jointoperation of the two separate measuring devices. Joint measurements withother measuring devices are typically not taken into account in thedesign of the measuring devices, wherein it is difficult to implementthe centralized control of the different measuring devices. Furthermore,a problem may be presented by particle losses in the transfer pipesystem between the different measuring devices.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a novel device for themeasurement of the size distribution of aerosol particles to overcomethe above-presented problems of the solution of prior art. This isachieved by connecting an electric mobility analyzer and an impactor toeach other in such a way that the bottom plate of the mobility analyzeris simultaneously used as the nozzle part of the first stage of theimpactor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail withreference to the appended drawings, in which

FIG. 1 shows an electric mobility analyzer according to prior art,

FIG. 2 shows an electric impactor according to prior art, and

FIG. 3 shows a measuring device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 havebeen discussed above in connection with the description of prior art.

FIG. 3 shows a device 30 for measuring the size distributions of aerosolparticles according to the invention. The device 30 combines a mobilityanalyzer 10 and an impactor 20 into one device. As described above inconnection with the operation of the mobility analyzer, the solution ofthe invention introduces a flow 11 to be analyzed via an inlet pipe 18into the mobility analyzer 10. In the above-described manner, particleswith a given electric mobility hit the inner electrode 13 of themobility analyzer 10.

FIG. 3 shows a solution based on the above-mentioned multi-channelmobility analyzer, in which the inner electrode 13 of the mobilityanalyzer 10 is equipped with several measuring surfaces. However, thesolution of the invention is not limited solely to said analyzersolution, but the mobility analyzer belonging to the device 30 accordingto the invention can also be implemented in another way, for examplewith a solution similar to that shown in FIG. 1, in which particlesfalling into a given mobility range are guided through a slit 19 into asecond outlet pipe 14, the main flow 11 continuing its travel past theopening 19.

In the device 30 shown in FIG. 3, the flow passed through the mobilityanalyzer 10 is immediately guided into the impactor 20 coupled after themobility analyzer 10. The coupling between the mobility analyzer 10 andthe impactor 20 is preferably implemented so that the bottom plate 17 bof the mobility analyzer 20, shown in FIG. 1, is the nozzle part 22 a ofthe first stage of the impactor 20, shown in FIG. 2.

The impactor 20 of FIG. 3 comprises three stages 23 a, 23 b and 23 cwhich are electrically separated by means of insulators 24 a, 24 b, 24c, 24 d from each other and from the mobility analyzer 10.

The presented solution makes it possible to merge the mobility analyzer10 and the impactor 20 into one compact analysis device 30. For example,the devices to be used for the control and calibration of the devices,such as the means required for adjusting the voltages of the mobilityanalyzer 10, can be centrally placed in a single control unit 28.Furthermore, the mobility analyzer 10 and the impactor 20 being clearlyintegrated in a single unit, the devices can be easily designed tosupport each other from the beginning, wherein the calibration andcontrol of the integrated device can be implemented in a considerablysimpler way than in the case of two separate measuring devices.

The mobility analyzer 10 and the impactor 20 can be advantageouslydesigned in such a way that the mobility analyzer 10 is used to measureparticles smaller than a given mobility-size distribution value, and theimpactor 20 is used to measure particles larger than a given aerodynamicdiameter value. The above-mentioned values can be preferably determinedso that the mobility analyzer measures within the particle size rangewhere it is more accurate than the impactor, and the impactor measureswithin a particle size range where it has a better measurement accuracythan the mobility analyzer. Naturally, the measurement ranges may alsobe partially overlapping.

Thanks to the solution of the invention, possible losses in the transferpipe between the devices are also eliminated, which increases thereliability of the measurement, compared with the solution of twoseparate analyzers placed one after the other.

The solution of the invention is particularly well suitable formeasurements of size distribution in real time, but it is not, however,solely limited to this solution, because the solution of the inventionis also applicable in so-called integrating measurements (laittaisinnäin vaikka vaikka alkutekstissälukeekin “mittaukseen”), in whichparticles to be measured are deposited in the analyzer for a certainperiod of time. After depositing the particles, the particlesaccumulated during the whole measurement period are measured.

The solution according to the invention is not limited solely to theabove-described examples, but it may vary within the scope of theinvention. In particular, the invention is not limited to the mobilityanalyzer and impactor types used in the examples, but the solution ofthe invention can be implemented with various mobility analyzer andimpactor types, known as such.

Naturally, it will be obvious for anyone skilled in the art that theterm “bottom plate” of the mobility analyzer, when used in thedescription and in the appended claims, generally refers to that part ofthe mobility analyzer through which the flow exits the analyzer. Saidterm should not be interpreted in a limited sense in such a way that itwould always be the lowermost part of the mobility analyzer. Ifnecessary, the bottom part may also comprise a design different from theplate-like shape, and protruding parts, etc., to implement the device ofthe invention.

1. A device for determining the size distribution of aerosol particlesfrom a flow, comprising: an electric mobility analyzer operative toreceive the flow; and an impactor comprising a first stage including anozzle part, wherein the impactor is operative to receive the flow fromthe electric mobility analyzer and is operatively connected to themobility analyzer in such a way that the nozzle part of the first stageof the impactor also forms a bottom plate of said mobility analyzer, andsaid mobility analyzer and said impactor are designed so that saidmobility analyzer collects from the flow particles smaller than a givenmobility diameter value, and said impactor collects from the flowparticles larger than a given aerodynamic diameter value, and whereinsaid mobility diameter value is such that said mobility analyzeroperates on a particle size range where a measurement accuracy of saidmobility analyzer is better than a measurement accuracy of said impactorand wherein said impactor measures particles within a particle sizerange where the impactor has a better measurement accuracy that themobility analyzer.
 2. The device according to claim 1, wherein saidimpactor is an electrical low pressure impactor.
 3. The device accordingto claim 1, wherein said mobility analyzer is a multi-channeldifferential mobility analyzer.
 4. The device according to claim 1,wherein substantially a whole flow passing through said mobilityanalyzer, excluding aerosol particles deposited in the mobilityanalyzer, is guided through the nozzle part of the first stage of theimpactor.
 5. The device according to claim 1, wherein said mobilitydiameter value and said aerodynamic diameter value correspond to thesame particle size.
 6. A device for determining the size distribution ofaerosol particles from a flow to be analyzed, comprising: an electricmobility analyzer including an inlet for the flow; and an impactorcomprising a first stage including a nozzle part including a guideoperative to guide the flow from the mobility analyzer to the impactor,wherein the impactor is operatively connected to the mobility analyzerin such a way that the nozzle part of the first stage of the impactoralso forms a bottom plate of said mobility analyzer, and said mobilityanalyzer and said impactor are designed so that said mobility analyzercollects particles smaller than a given mobility diameter value, andsaid impactor collects particles larger than a given aerodynamicdiameter value, and wherein said mobility diameter value is such thatsaid mobility analyzer operates on a particle size range where ameasurement accuracy of said mobility analyzer is better than ameasurement accuracy of said impactor and wherein said impactor measuresparticles within a particle size range where the impactor has a bettermeasurement accuracy that the mobility analyzer.
 7. The device accordingto claim 1, wherein said aerodynamic diameter value is such that saidimpactor operates on a particle size range where the measurementaccuracy of said impactor is better than the measurement accuracy ofsaid mobility analyzer.
 8. The device according to claim 7, wherein saidparticle ranges where said impactor and said mobility analyzer operateare partially overlapping.